Android build files

[jb55]

Instead of using cargo-apk in

• https://github.com/damus-io/android/issues/74

We use a direct android gradle build for building the app. This should give us a bit more flexibility when building our app instead of letting cargo-apk/xbuild handle it.

It also allows us to establish a GameActivity build directly for:

• Switch to GameActivity #190

Example

• https://github.com/podusowski/walkers/tree/main/demo_android